1. Field of the Invention
This invention relates to an ink for use in capillary type writing instruments, and more particularly, to aqueous ink compositions promoting rejuvenation after the writing tip of a capillary type writing instrument has dried out from exposure to the atmosphere.
2. Description of the Prior Art
A problem frequently encountered by people who use capillary type writing instruments is writing point dryout. Capillary type writing instruments include those pens or marking devices which have fiber, felt, or porous plastic type writing points which deliver ink from a reservoir to the writing point by means of capillary action. The inks used in capillary type writing instruments have generally been of two types: (1) water based inks containing water soluble dyes, (the inks of the present invention), and (2) inks which contain organic solvents and dyes which are soluble in an organic medium.
Capillary type writing instruments employ a variety of types of fiber, felt, and porous plastic writing points. Felt is a fabric of natural and/or synthetic fibers worked together by pressure, heat, or chemical action without weaving or knitting. The fibers tend to be arranged in a random fashion. When felt is used as a writing point, the ink is conveyed through the interstices between the individual fibers by capillary action.
A second major type of writing point is made by bonding together a bundle of parallel strands of either natural or synthetic fibers (e.g. polyvinyl chloride, polyethylene, or polyamide polymers) to form capillary pathways between the bundles leading to the writing point. U.S. Pat. No. 3,558,392 is an example of such a point. The fibrous point is made by setting a bundle of parallel strands of synthetic fibers in a desired relation by drawing the bundle through a heat-setting zone to establish dimensions in cross-section of the final object and to create longitudinally extending channels. The strength of the point is subsequently increased by immersion in a dilute resin solution followed by heating to evaporate the solvent and cure the resin. Depending on the denier of the fibers of the particular yarn which is used to form the writing point, a number of channels will be formed having average or mean transverse dimensions of from 1.times.1 mils. to 3.times.6 mils. Channels of 1.5 to 3 mils. in one transverse direction and between 3 and 6 mils. in another direction are representative of the most desirable channel sizes. Thus, the capillary channels are typically between 4.5 and 18 sq. mils. in cross-sectional area.
An example of another type of writing point is described in U.S. Pat. No. 3,778,496. The point described there is an elongated unitary writing element made by extruding and meltdrawing a polymer (e.g. acrylic, acetal, polyamide, or polyester polymers) and then reforming the internal portions of the extruded body by a compressive drawing operation to obtain channels of a desired size. The writing point thus formed has a longitudinally extending, axial, ink-conveying channel and circumaxial, spaced, ink-conveying channels, the axial channel having a cross-sectional area equivalent to a circle with a diameter between two and four mils. The enlarged circumaxial ink-conveying channels have an oblong cross-sectional shape approximately 1 to 3 mils. wide and 3 to 5 mils. in a radial direction. The circumaxial ink-conveying channels are connected to the axial channel by capillary channels having diameters of between 0.4 mils. and 1.0 mils. The longitudinal channels of the rods serve to deliver ink to the writing point by means of capillary action.
In each of the above cases, the ink is fed from a reservoir to the writing surface by means of the writing point. The reservoir which feeds the writing point may be either a container of liquid ink or a bundle of fibers saturated with ink. Either will perform equally in capillary writing instruments since dryout is attributable to changes occuring at the writing tip rather than inside the pen.
A problem with capillary type writing instruments is that when exposed to the atmosphere, as when left uncapped for a period of time, the water in the ink may evaporate so as to change the composition of the ink and make it more viscous. The capillaries within the tip of the writing instrument will then become clogged so that the written line produced becomes irregular. If left uncapped long enough, the composition of the ink at the tip may be so changed that it will not flow at all through the capillaries of the tip, writing becomes impossible, and the instrument must be discarded.
Attempts have been made to retard the rate at which water based inks dry out by adding humectant materials such as ethylene glycol, propylene glycol, and other higher molecular weight polyhydroxy compounds such as those described in U.S. Pat No. 3,705,045. However, such materials tend to increase the viscosity of the ink to such an extent that it will not flow freely through the capillaries of capillary type writing instruments. Furthermore, the selective evaporation of the more volatile constituents of the ink may cause the inks to be thickened at the exposed tip of the instrument by the less volatile polyhydroxy compounds which remain. If additional polyhydroxy compound is used in the ink formulation, the gains from increased humectancy are lost to the increasing viscosity.
In an attempt to solve the point dryout problem, U.S. Pat. No. 3,519,443, describes inks formulated which formamide as a solvent to reduce the rate of point dryout. Formamide is somewhat toxic. This constitutes a disadvantage since capillary type writing instruments containing water-based inks may be used by children. In addition, formamide is suggested for use at concentrations from 10% to 100% of the total solvent content of the ink, levels at which a disagreeable odor may be produced.
In spite of the above attempts to reduce point dryout, problems still exist. Therefore, a need exists for ink compositions which not only reduce the rate of dryout, but allow rejuvenation even after dryout has occurred.